This invention relates to internally crosslinked polymer microparticles having a three-dimensional network structure.
Recently, internally crosslinked polymer microparticles commonly known as polymer microgels have become interested in the coating industry. The provision of high-solids coating compositions has been demanded for social reasons from the viewpoint of saving natural resources and pollution control. Polymer microgels may be effectively used in the preparation of high-solids coating compositions without compromising their workability. Besides high-solids coating compositions, polymer microgels find a wide variety of uses such as adhesives, sealants, optical fiber coverings, printing materials, biomedical materials and the like.
Several methods are known to produce polymer microgels. One such method includes the steps of emulsion polymerizing a mixture of ethylenically unsaturated monomers including at least one crosslinking comonomer in an aqueous medium, and then removing water from the resulting polymer emulsion by, for example, solvent substitution, azeotropic distillation, centrifugation, filtering or drying.
Japanese Patent Application Nos. 56/71864 and 57/13052 disclose an emulsion polymerization method using, as an emulsifier and dispersant, a compound or resin having an amohoionic group of the formula: ##STR1## wherein R is optionally substituted alkylene or phenylene and Y is --COOH or --SO.sub.3 H. The compound or resin having such amphoionic group will be bound into the polymers constituting microgels physically or through covalent bonds. The use of said compound or resin as an emulsifier and dispersant is advantageous in that it can dispense with the step of subsequently removing emulsifier or dispersant as required when using conventional surfactants which, if remained, will adversely affect properties of coating films. Furthermore, the microgels exhibit a number of advantageous characteristics such as high stability and dispersibility in both aqueous and nonaqueous systems based on the presence of said amphoionic groups.
Experiments have shown, however, that polymer microparticles bearing said amphoionic groups become unstable against pH variation, particularly in acidic ranges. This is because, whereas said amphoionic group serves to increase the stability of polymer dispersions in water in the presence of a sufficient amount of counter ions, such counter ions are entrapped by another compound when pH has varied.
Experiments have also shown that when a compound or resin having said amphoionic group of the formula: ##STR2## is used in the emulsion polymerization to prepare internally crosslinked polymer microparticles, the proportion of polyfunctional crosslinking monomers is usually limited upto 20% of total monomer mixtures. When higher solvent resistance, weatherability, thermal deformation resistance and other physical and chemical properties are desired for microgels, the crosslinking density of microgels must be increased as high as possible by increasing the proportion of the crosslinking monomers to 50-100%.